1. Field of the Invention
The present invention is directed to a drive system for a vehicle comprising an internal combustion engine and a torsional vibration damping arrangement coupled to a crankshaft of the internal combustion engine.
2. Related Art
Internal combustion engines used for driving vehicles have an irregular, particularly an oscillating, torque curve due to the combustion process and the ignitions occurring substantially periodically in the cylinders thereof. A vibration component is superposed on a nominal torque, wherein the vibration frequency depends on the combustion process, that is, on whether a two-stroke process or a four-stroke process is used, and on the quantity of cylinders. For example, in a four-cylinder four-stroke engine, an ignition occurs in two cylinders for every revolution of its crankshaft so that there are two excitation events per revolution of the crankshaft. Therefore, as regards the rotational speed of the crankshaft, the second order is critical because the ignition frequency in an internal combustion engine of this type corresponds to twice the rotational speed of the crankshaft. Correspondingly, in a three-cylinder four-stroke engine the 1.5th order is problematic with respect to vibrations, whereas in a four-cylinder two-stroke engine, the second order is problematic, again with respect to the rotational speed of the crankshaft.
In order to eliminate these excitations of vibrations, or to prevent them as far as possible from being transmitted into subsequent system areas of a drivetrain, torsional vibration damping arrangements are used that are tuned to the vibration excitation behavior in the drivetrain, particularly in the region of the internal combustion engine.
In modern internal combustion engines, it is possible to provide the capability of switching between different operating modes. Thus, for example, an internal combustion engine having multiple cylinders can be operated in such a way that all of the cylinders are active so that it is capable of providing maximum torque under full load conditions. If this is unnecessary, it is possible to change to an operating mode in which only some of the cylinders are active. In this case, the cylinders that remain active under a partial load condition are loaded to a greater degree, which generally entails the advantage that the internal combustion engine and the individual cylinders thereof can work with greater efficiency than under reduced load. This has an advantageous impact on consumption and pollutant emission. In principle, it is also possible to switch between different combustion processes, i.e., between two-stroke operation and four-stroke operation, and it is also possible in this case to switch between operating modes of different performance with high performance capacity in two-stroke operation, and the internal combustion engine can operate in four-stroke mode with a reduced performance requirement.
However, switching between different operating modes with different performance, i.e., particularly switching different cylinders on or off or changing between different combustion processes, has a substantial influence on the vibration excitation behavior in an internal combustion engine. For example, if half of the cylinders are deactivated in an internal combustion engine operating in four-stroke mode, the critical excitation order is also halved. For example, when two cylinders are switched off in a four-cylinder four-stroke engine, only one ignition takes place per revolution, with the result that the critical excitation order—again with respect to the rotational speed of the crankshaft—is now of the first order and no longer of the second order. But with this change in the vibration excitation behavior, the vibration damping behavior of a torsional vibration damping arrangement likewise changes. If this torsional vibration damping arrangement is tuned to a particular excitation spectrum, a shift in the excitation spectrum, particularly in the excitation order, can lead to a reduced vibration damping capability so that, as a result, rotational irregularities generated in the region of the internal combustion engine, that is, particularly vibration components in the torque, are transmitted to a greater extent to areas of the drivetrain downstream in the torque path.